Hypertension is detrimental to the brain and appears to present a clearly identifiable risk factor in the profile of the stroke-prone individual. Increased blood pressure may alter brain function by acceleration of cerebral atherosclerosis or by initiating a series of pathologic changes in small arteries (lipohyalinosis). The results of these vascular derangements appear to be parenchymal infarctions, commonly encountered in both normotensive and hypertensive patients, and intracerebral hemorrhages and small lacunar infarcts, which are almost exclusively confined to hypertensive brains. Physiologic, pathologic, clinical, and epidemiologic data support the thesis that treatment of hypertension is associated with decreased morbidity and a decrease in the incidence of subsequent cerebrovascular disease.